Leucite glass ceramics

Abstract

A leucite glass-ceramic is prepared from a glass comprising: about 66.8 to about 71.9 mol % of SiO.sub.2, about 8.5 to about 10.6 mol % of Al.sub.2O.sub.3, about 9.5 to about 12.8 mol % of K.sub.2O, about 0.5 to about 4.0 mol % of CaO, about 0 to about 3.0 mol % of TiO.sub.2, about 1.8 to about 4.0 mol % of Na.sub.2O, about 0.1 to about 6.0 mol % of Li.sub.2O, about 0 to about 1.0 mol % of MgO, about 0 to about 3.0 mol % of Nb.sub.2O.sub.5, and about 0 to about 3.0 mol % of B.sub.2O.sub.3. The leucite glass-ceramic is prepared by subjecting the glass components to a nucleation heat treatment, followed by a growth heat treatment. The leucite glass-ceramic may be used in the fabrication of a Dental restoration using various processes, and may be used in the construction of Dental restorations such as ceramic Dental inlays, crowns, veneers, bridges, veneering materials for zirconium oxide restoration substrates, alumina oxide restoration substrates, or metal restoration substrates.

Claims

1. A leucite glass-ceramic prepared from a glass comprising: 66.8 to 71.9 mol % of SiO.sub.2, 8.5 to 10.6 mol % of Al.sub.2O.sub.3, 9.5 to 12.8 mol % of K.sub.2O, 0.5 to 4.0 mol % of CaO, 0 to 3.0 mol % of TiO.sub.2, 1.8 to 4.0 mol % of Na.sub.2O, 0.1 to 6.0 mol % of Li.sub.2O, 0 to 1.0 mol % of MgO, 0 to 3.0 mol % of Nb.sub.2O.sub.5, and 0 to 3.0 mol % of B.sub.2O.sub.3.

2. A leucite glass-ceramic according to claim 1, wherein the leucite glass-ceramic is prepared from a glass comprising: 66.8 to 71.9 mol % of SiO.sub.2, 8.5 to 10.3 mol % of Al.sub.2O.sub.3, 9.5 to 12.8 mol % of K.sub.2O, 0.5 to 4.0 mol % of CaO, 0 to 3.0 mol % of TiO.sub.2, 1.9 to 4.0 mol % of Na.sub.2O, 0.1 to 2.0 mol % of Li.sub.2O, 0 to 1.0 mol % of MgO, 0 to 3.0 mol % of Nb.sub.2O.sub.5, and 0 to 3.0 mol % of B.sub.2O.sub.3.

3. A leucite glass-ceramic according to claim 1, wherein the K.sub.2O content is from 10.0 to 12.8 mol %.

4. A leucite glass-ceramic according to claim 1, wherein the CaO content is from 0.5 to 3.0 mol %.

5. A leucite glass-ceramic according to claim 1, wherein the TiO.sub.2 content is less than 0.1 mol %.

6. A leucite glass-ceramic according to claim 1, wherein the Li.sub.2O content is from 0.1 to 1.6 mol %.

7. A leucite glass-ceramic according to claim 1, wherein the MgO content is from 0 to 0.5 mol %.

8. A leucite glass-ceramic according to claim 1, wherein the Nb.sub.2O.sub.5 content is from 0.5 to 2.0 mol %.

9. A leucite glass-ceramic according to claim 1, wherein the B.sub.2O.sub.3 content is less than 0.1 mol %.

10. A leucite glass-ceramic according to claim 1, wherein the glass-ceramic is prepared from a glass comprising approximately equal molar percentages of CaO and TiO.sub.2.

11. A leucite glass-ceramic according to claim 1, wherein the glass-ceramic displays high flexural strength of at least 200 MPa.

12. A leucite glass-ceramic according to claim 1, wherein the morphology of the crystals is mainly in the form of fibers, rosettes, and/or spheres.

13. A leucite glass-ceramic according to claim 1, wherein the glass-ceramic is translucent.

14. A leucite glass-ceramic according to claim 1, wherein the refractive index of the glass-ceramic and/or the thermal properties of the glass-ceramic are matched to the leucite phase.

15. A leucite glass-ceramic according to claim 1, wherein the glass-ceramic displays one or more of the following properties: (1) a leucite area fraction of from 33.1 to 65.7; (2) a glass coefficient of thermal expansion (CTE) of from 8.510.sup.6/K to 10.710.sup.6/K at 100-400 C.; and (3) a glass-ceramic coefficient of thermal expansion (CTE) of from 15.710.sup.6/K to 24.910.sup.6/K at 100-400 C.

16. A method of preparing a leucite glass-ceramic according to claim 1 comprising subjecting the glass to a nucleation heat treatment followed by a growth heat treatment.

17. A method of preparing a leucite glass-ceramic according to claim 1 comprising subjecting the glass to a nucleation heat treatment at a temperature of 587 C. to 670 C. for a duration of about 0.5 hours to 4 hours, followed by a growth heat treatment at a temperature of 795 C. to 1075 C. for a duration of 0.5 hours to 3 hours.

18. A method of preparing a leucite glass-ceramic according to claim 16, wherein: (a) the leucite glass-ceramic is crystallised via a bulk crystallisation mechanism, or (b) the leucite glass-ceramic is crystallised via a surface crystallisation mechanism.

19. A method of fabricating a Dental restoration comprising the leucite glass-ceramic according to claim 1, the method comprising one or more of the following processes: sintering, heat pressing, Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) technology, and 3D printing technology.

20. The method of claim 19, wherein the Dental restoration is selected from the group consisting of: a ceramic Dental inlay, a crown, a veneer, a bridge, and a veneering material for a zirconium oxide restoration substrate, an alumina oxide restoration substrate, and a metal restoration substrate.

21. A Dental restoration selected from the group consisting of: a ceramic Dental inlay, a crown, a veneer, a bridge, and a veneering material for a zirconium oxide restoration substrate, an alumina oxide restoration substrate, and a metal restoration substrate, comprising a leucite glass-ceramic according to claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the .sup.27Al MAS NMR spectra for glass-ceramics of the invention.

(2) FIG. 2 shows the DSC traces of the glass of Example 1 at a rate of 20 C./minute.

(3) FIG. 3 shows the .sup.27Al triple-quantum MAS NMR spectrum of the glass-ceramic of Example 1a.

(4) FIG. 4 shows the SEM photomicrograph images of the glass-ceramic of Example 1 with orientated fiber-like crystals.

(5) FIG. 5 shows the SEM photomicrograph images of the re-homogenized glass-ceramic of Example 1a with fine leucite fibers (refined microstructure).

(6) FIG. 6 shows the SEM photomicrograph of the re-homogenized glass-ceramic of Example 1a after a secondary heat treatment showing ripening of the leucite crystals.

(7) FIG. 7 shows the SEM photomicrograph of the re-homogenized glass-ceramic of Example 1a after heat extrusion.

(8) FIG. 8 shows the SEM photomicrograph of the glass-ceramic of Example 2a showing a uniform microstructure of largely rounded leucite crystals.

(9) FIG. 9 shows the SEM photomicrograph of the glass-ceramic of Example 2b showing an increase of high aspect ratio leucite crystals in the glassy matrix.

(10) FIG. 10 shows the SEM photomicrograph of the glass-ceramic of Example 2c showing high aspect leucite crystals, with signs of cross-shaped geometry in the glassy matrix.

(11) FIG. 11 shows the SEM photomicrograph of the glass-ceramic of Example 2c showing high aspect leucite crystals, with signs of cross-shaped geometry in the glassy matrix.

(12) FIG. 12 shows the SEM photomicrograph of the glass-ceramic of Example 3 showing a high area fraction of leucite crystals in the glassy matrix.

(13) FIG. 13 shows the SEM photomicrograph of the glass-ceramic of Example 3a showing a high area fraction of leucite crystals with some high aspect ratio leucite crystallites.

(14) FIG. 14 shows the XRD plots for the glass-ceramic of Examples 1 and 3 demonstrating the presence of tetragonal leucite.

(15) FIG. 15 shows the SEM photomicrograph of the glass-ceramic of Example 4 showing a high area fraction of leucite crystals with some elongated leucite crystallites.

(16) FIG. 16 shows the SEM photomicrograph of the glass-ceramic of Example 4a showing a high area fraction of spherical leucite crystals uniformly distributed in the glassy matrix.

(17) FIG. 17 shows the high temperature XRD patterns for the glass-ceramic of Example 4 indicating the presence of sanidine and leucite at different crystallisation temperatures.

(18) FIG. 18 shows the SEM photomicrograph of the glass-ceramic of Example 4 showing a high area fraction of fine spherical leucite crystals uniformly distributed in the glassy matrix.

(19) FIG. 19 shows a first SEM image of the leucite glass-ceramic of Example 5 and a second SEM image of the leucite glass-ceramic of Example 5 after 620/2 hr and 920/1 hr holds.

(20) FIG. 20 shows two SEM images of the glass-ceramic frit of Example 6.

(21) FIG. 21 shows a first SEM photomicrograph of the glass-ceramic of Example 7 and a second SEM photomicrograph of the glass-ceramic of Example 7 following a 30-minute high speed milling process showing the change in microstructure.

DETAILED DESCRIPTION OF THE INVENTION

(22) The present invention provides a leucite glass-ceramics with high flexural strength, high reliability, minimal microcracking, controlled translucency, and/or controlled volume fraction of leucite crystals, the properties being achieved by matching the refractive index of the glass-ceramic to the leucite phase, and/or matching the thermal properties of the glass-ceramic to the leucite phase.

(23) The following definitions shall apply throughout the specification and the appended claims.

(24) Within the context of the present specification, the term comprises is taken to mean includes or contains, i.e. other integers or features may be present, whereas the term consists of is taken to mean consists exclusively of.

(25) Within the present specification, the term about means plus or minus 20%; more preferably plus or minus 10%; even more preferably plus or minus 5%; most preferably plus or minus 2%.

(26) The present invention employs a novel strategy for developing leucite glass-ceramics with particularly advantageous properties through the use of Appen factors to predict the glass properties using the following equation:

(27) $={}_1{p}_1+{}_2{p}_2+\mathrm{.Math.}+{}_n{p}_n=\underset{i=1}{\overset{n}{.Math.}}{}_i{p}_i$
where ai indicates the properties contribution of each oxide to glass, and Pi is the weight fraction of each oxide.

(28) The various properties (density, thermal expansion coefficient, refractive indices, fusion temperature) for each of the oxide components for use in the above Appen factor equation are as follows:

(29) TABLE-US-00001 Appen Factors Thermal Fusion Expansion Temperature Coefficient ( C., (TEC) Winkelmann Oxide Density (10.sup.6/K, Refractive and component .sup..sup.i (g/cm.sup.3) 20-400 C.) Index (R.I.) .sup.n.sup.i Schott) SiO.sub.2 26.1-27.25 0.5-3.8 1.4585-1.475 6.161 Al.sub.2O.sub.3 40.4 3.0 1.520 34.585 K.sub.2O 34.1 (33.5) 46.5 (42.0) 1.575 (1.595) 783.92 CaO 14.4 13.0 1.730 885.046 TiO.sub.2 20.5 1.5-3.0 2.080-2.230 79.806 Na.sub.2O 20.2 39.5 1.590 670.233 Li.sub.2O 11.0 27.0 1.695 MgO 12.5 6.0 1.610 1125.698 For some Appen Factors, the following remarks apply: SiO.sub.2: when 67 p.sub.SiO.sub.2 100, .sub.SiO.sub.2 = 10.5 0.1 .Math. p.sub.SiO.sub.2, .sub.SiO.sub.2 = 23.75 + 0.035 .Math. p.sub.SiO.sub.2 and n.sub.SiO.sub.2 = 1.5085 0.0005 .Math. n.sub.SiO.sub.2, when p.sub.SiO.sub.2 67, .sub.SiO.sub.2 = 26.1, .sub.SiO.sub.2 = 3.8, and n.sub.SiO.sub.2 = 1.475. K.sub.2O: the values in parentheses only apply to glass with composition of p.sub.Na.sub.2.sub.O < 1% , TiO.sub.2: when 50 p.sub.SiO.sub.2 80, .sub.TiO.sub.2 = 10.5 0.15 .Math. p.sub.SiO.sub.2 and .sub.TiO.sub.2 = 2.480 0.005 .Math. p.sub.SiO.sub.2.

(30) The weight percentage of oxides p.sub.i is expressed in mole percentage (Scholze and Lakin, 1991).

(31) Using the glass prediction results, a range of glasses was designed using glass reagents with compositions listed in Table IA in the form of silicates, carbonates and oxides.

(32) TABLE-US-00002 TABLE IA Materials Mole Percentage SiO.sub.2 66.8-71.9 Al.sub.2O.sub.3 8.5-10.6 K.sub.2O 9.5-12.8 CaO 0.5-4.0 TiO.sub.2 0-3.0 Na.sub.2O 1.8-4.0 Li.sub.2O 0.1-6.0 MgO 0-1.0 Nb.sub.2O.sub.5 0-3.0 B.sub.2O.sub.3 0-3.0

(33) Accordingly, the present invention provides a leucite glass-ceramic prepared from a glass comprising: about 66.8 to about 71.9 mol % of SiO.sub.2, about 8.5 to about 10.6 mol % of Al.sub.2O.sub.3, about 9.5 to about 12.8 mol % of K.sub.2O, about 0.5 to about 4.0 mol % of CaO, about 0 to about 3.0 mol % of TiO.sub.2, about 1.8 to about 4.0 mol % of Na.sub.2O, about 0.1 to about 6.0 mol % of Li.sub.2O, about 0 to about 1.0 mol % of MgO, about 0 to about 3.0 mol % of Nb.sub.2O.sub.5, and about 0 to about 3.0 mol % of B.sub.2O.sub.3.

(34) Using the glass prediction results, another range of glasses were designed using glass reagents with compositions listed in Table IB in the form of silicates, carbonates and oxides.

(35) TABLE-US-00003 TABLE IB Materials Mole Percentage SiO.sub.2 66.8-71.9 Al.sub.2O.sub.3 8.5-10.3 K.sub.2O 9.5-12.8 CaO 0.5-4.0 TiO.sub.2 0-3.0 Na.sub.2O 1.9-4.0 Li.sub.2O 0.1-2.0 MgO 0-1.0 Nb.sub.2O.sub.5 0-3.0 B.sub.2O.sub.3 0-3.0

(36) Accordingly, the present invention also provides a leucite glass-ceramic prepared from a glass comprising: about 66.8 to about 71.9 mol % of SiO.sub.2, about 8.5 to about 10.3 mol % of Al.sub.2O.sub.3, about 9.5 to about 12.8 mol % of K.sub.2O, about 0.5 to about 4.0 mol % of CaO, about 0 to about 3.0 mol % of TiO.sub.2, about 1.9 to about 4.0 mol % of Na.sub.2O, about 0.1 to about 2.0 mol % of Li.sub.2O, about 0 to about 1.0 mol % of MgO, about 0 to about 3.0 mol % of Nb.sub.2O.sub.5, and about 0 to about 3.0 mol % of B.sub.2O.sub.3.

(37) It has surprisingly been found that the K.sub.2O content has an effect on the morphology of the crystals. Higher contents of K.sub.2O tend to result in glass-ceramics with higher proportions of spherical crystals which are useful in certain applications. Accordingly, in one embodiment, the K.sub.2O content is from about 10.0 to about 12.8 mol % Lower contents of K.sub.2O tend to result in glass-ceramics with higher proportions of crystals in the form of elongated fibers, which have a number of advantages in terms of strength and machinability in certain applications. Accordingly, in another embodiment, the K.sub.2O content is from about 9.5 to about 10.0 mol %.

(38) In a preferred embodiment of the invention, the CaO content is from about 0.5 to about 3.0 mol %.

(39) It has surprisingly been found that the leucite glass-ceramics of the present invention can be produced with or without titanium dioxide nucleating agents. Previously, it was thought that the presence of titanium dioxide was essential for effective crystallization of leucite to occur. Accordingly, in one embodiment of the invention, the TiO.sub.2 content is less than about 0.1 mol %; more preferably, no titanium dioxide is used.

(40) In one preferred embodiment of the invention, the Li.sub.2O content is from about 0.1 to about 2.0 mol %. In a more preferred embodiment of the invention, the Li.sub.2O content is from about 0.1 to about 1.6 mol %.

(41) In an alternative preferred embodiment of the invention, the Li.sub.2O content is from about 1.6 mol % to about 6.0 mol %; more preferably, from about 2.0 mol % to about 4.0 mol %; most preferably, from about 2.0 mol % to about 6.0 mol %. It has surprisingly been found that utilising a higher percentage of Li.sub.2O results in a lowering of the temperature of manufacture, resulting in lower manufacturing costs, improved ease of manufacturing, and reduced safety risks of manufacturing.

(42) In a preferred embodiment of the invention, the MgO content is from about 0 to about 0.5 mol %. More preferably, the MgO content is from about 0.1 to about 0.5 mol %. In a preferred embodiment of the invention, the Nb.sub.2O.sub.5 content is from about 0.5 to about 2.0 mol %.

(43) It has surprisingly been found that the leucite glass-ceramics of the present invention can be produced with or without boric oxide as a glass component, whereas it was previously considered that this was an essential component for producing leucite glass-ceramics. Accordingly, in one embodiment of the invention, the B.sub.2O.sub.3 content is less than about 0.1 mol %; more preferably, no boric oxide is used.

(44) It has surprisingly been found that the calcium oxide and titanium dioxide components have a synergistic effect when used in approximately equal molar percentages. Leucite glass-ceramics prepared from glasses comprising approximately equal molar percentages of calcium oxide and titanium dioxide have been shown to produce tetragonal leucite glass-ceramics with high reliability, and with varying microstructures. Accordingly, in one embodiment of the invention, the glass-ceramic is prepared from a glass comprising approximately equal molar percentages of CaO and TiO.sub.2. In this embodiment, the CaO content is preferably from about 0.5 to about 3.0 mol %, and the TiO.sub.2 content is preferably from about 0.5 to about 3.0 mol %.

(45) Glasses were designed to attain thermal expansion coefficients ranging from about 8.510.sup.6/K to about 10.710.sup.6/K (Table II) and with calculated refractive indices (RI) similar to the leucite phase (natural leucite RI=1.508-1.511). Accordingly, in one embodiment of the invention, the leucite glass-ceramic has a refractive index that is matched to the leucite phase and/or the leucite glass-ceramic has thermal properties that are matched to the leucite phase. In another embodiment of the invention, the leucite glass-ceramic displays one or more of the following properties: (1) a leucite area fraction of from about 33.1 to about 65.7%; (2) a glass coefficient of thermal expansion (CTE) of from about 8.510.sup.6/K to about 10.710.sup.6/K at 100-400 C.; and (3) a glass-ceramic coefficient of thermal expansion (CTE) of from about 15.710.sup.6/K to about 24.910.sup.6/K at 100-400 C. Preferably, the refractive index of the leucite glass-ceramic is such that the glass-ceramic being translucent.

(46) The fusion temperature contribution of each oxide (Winkelmann and Schott, 1894) was also taken into account to provide an estimate of the likely processing temperatures available. Processing temperatures were calculated to ensure they were in the range suitable for dental restoration manufacturing parameters.

(47) It has been found by the present inventors that high flexural strength (i.e. at least about 200 MPa) is achieved when the maximum aluminium present in the novel glass composition is consumed by the crystallising leucite phase and minimum aluminium remains in the glass matrix. Accordingly, in one embodiment of the invention, the leucite glass-ceramic displays high flexural strength of at least about 200 MPa.

(48) Leucite glass-ceramics of the present invention provide a range of controllable morphologies which may be useful for different specific applications. Accordingly, in one embodiment of the invention, the morphology of the crystals of the leucite glass-ceramic is mainly in the form of fibers, rosettes, and/or spheres.

(49) Leucite glass-ceramics of the present invention may be prepared by any suitable method known to the skilled person.

(50) In one embodiment of the invention, there is provided a method of preparing a leucite glass-ceramic of the invention comprising the steps of: (a) preparing a glass from the glass components specified above, and (b) subjecting the glass to a nucleation heat treatment followed by a growth heat treatment. Preferably, the nucleation heat treatment comprises heating at a temperature of about 587 C. to about 670 C. Preferably, the nucleation heat treatment is applied for a duration of about 0.5 hours to about 4 hours. Preferably, the growth heat treatment comprises heating at a temperature of about 795 C. to about 1075 C. Preferably, the growth heat treatment is applied for a duration of about 0.5 hours to about 3 hours.

(51) In a further embodiment of the invention, there is provided a method of preparing a leucite glass-ceramic of the invention comprising subjecting the glass components to a nucleation heat treatment at a temperature of about 587 C. to about 670 C. for a duration of about 0.5 hours to about 4 hours, followed by a growth heat treatment at a temperature of about 795 C. to about 1075 C. for a duration of about 0.5 hours to about 3 hours. Using such a method, the leucite glass-ceramic may be crystallised via a bulk crystallisation mechanism or via a surface crystallisation mechanism.

(52) In another embodiment of the invention, there is provided a method of preparing a sanidine glass-ceramic or a leucite/sanidine glass-ceramic comprising subjecting the glass components as defined above to a nucleation heat treatment at a temperature of about 618 C. to about 620 C. for a duration of about 0.5 hours to about 4 hours, followed by a growth heat treatment at a temperature of about 720 C. to about 1075 C. for a duration of about 0.5 hours to about 3 hours.

(53) In order to prepare the leucite glass-ceramics of the invention, one specific procedure will now be described with reference to the Examples below. Raw materials are melted in an electrical chamber furnace (UAF1600 furnace, Lenton, Hope Valley, UK) at 1550 C. for about 5-6 hours (composition dependant, see Examples for representative conditions). Glass melts are next annealed at 500 C. for about 1 hour in a preheated furnace (Tris Burnout furnace, Dentalfarm, Italy), followed by furnace cooling. After cooling, the glass frits may be ball milled and sieved through a 125 micron sieve. In order to refine the microstructure, glasses may be remelted for about 2 hours at 1400-1550 C., and then annealed for about 2 hours at 500 C., reground, and then sieved through a 125 micron sieve. Glasses prepared using the method as described in the Examples below were transparent.

(54) Leucite glass-ceramics are produced by using a two-step heat treatment in all instances. Glass powders are heated in an electric furnace (RHF 1600, Carbolite, Bamford, UK) using different nucleation and crystal growth temperatures and holds (representative conditions are described in Examples 1-8). After the heat treatment, the glass-ceramics are air quenched. The glass frits are crushed and ground in a ball mill (Pascall Engineering ltd, UK) for about 2 hours then screened using a 125 m sieve.

(55) The area fraction and morphology of the leucite crystals can be controlled in the glass-ceramics of the invention depending on the composition and type of heat treatment applied (various representative conditions being described in Examples 1-8). It is possible to crystallise a high area fraction (>65%) of elongated fiber-like leucite crystals with high aspect ratio (as shown in Example 1), together with a high density of more spherical crystals. These elongated fiber-like crystals grow in preferred orientated domains. Re-homogenisation of these glasses can refine these crystallites (as demonstrated in Example 1, and FIG. 5). Leucite glass-ceramics are generally accepted as surface nucleated. A bulk or a surface crystallization process (depending on the formulation) can be taken advantage of in order to affect the microstructure, in terms of changes to the crystal morphology and/or area fraction. The leucite glass-ceramics of the invention can also be produced without titanium dioxide as a nucleating agent (as demonstrated in Example 3), whereas titanium dioxide was previously considered essential for the copious crystallization of leucite.

(56) The addition of TiO.sub.2 and CaO of equal molar % to a base formulation (as demonstrated in Examples 2, 2a, 2b, and 2c) can, however, be used to change the leucite morphology (crystallite aspect ratio) with an increase in strength (to about 224-234 MPa) and reliability (m=18.9). These values for reliability are higher than that found for traditional dental ceramics (Cattell et al., 1997, J Dent, 25; 409-414). These glass-ceramics attained a high coefficient of thermal expansion (19.6-24.9) with no glass matrix microcracking (as demonstrated in Example 2, 2a, 2b, 2c) which is linked to the improved mechanical performance.

(57) A structural factor linked to mechanical performance and evaluated via solid state nuclear magnetic resonance relates to the amount of aluminium remaining in the glass matrix after crystallization of leucite. To achieve high strength in the materials of the present invention, it is desirable to maximize consumption of aluminium by the leucite phase and minimize residual aluminium in the glass matrix as shown by .sup.27Al magic angle spinning nuclear magnetic resonance (MAS NMR). FIG. 1 shows .sup.27Al MAS NMR spectra for representative leucite glass-ceramics of the invention (Examples 1a, 5 and 6). The spectrum for the leucite glass-ceramic of Example 1a shows maximum consumption of aluminium into the leucite phase, with minimum aluminium remaining in the glass matrix.

(58) The degree of crystallization and crystal morphology can also be varied according to the crystallization heat treatment selection. The present invention provides leucite glass-ceramics with fibers, spherical or mixed morphologies, with resultant changes in the mechanical properties as is presented in Examples 1-8 (Table IV). It is also possible to change the microstructure of some of these glass-ceramic formulations by a short secondary processing heat treatment to produce a dense dispersal of more spherical type crystals (as demonstrated in Examples 1 and 1a) and a ripening of the leucite growth (as demonstrated in Example 1a). No microcracking was present in the glassy matrix of many of the Example leucite glass-ceramics of the present invention (as shown in FIGS. 4, 5 and 6) and a high flexural strength is produced (212.328 MPa, Table IV).

(59) Process for Producing Dental Restorations

(60) The leucite glass-ceramics of the present invention find particular application in the field of dentistry, and in particular, in the fabrication of dental restorations, such as inlays, crowns, veneers, bridges, and veneering materials for other restoration substrates.

(61) Accordingly, the present invention provides for the use of a leucite glass-ceramic of the invention or a sanidine glass-ceramic or a leucite/sanidine glass-ceramic prepared by the method of the invention in the construction of a Dental restoration selected from the group consisting of a ceramic Dental inlay, a crown, a veneer, a bridge, and a veneering material for a zirconium oxide restoration substrate, an alumina oxide restoration substrate, or a metal restoration substrate.

(62) The present invention also provides a Dental restoration selected from the group consisting of: a ceramic Dental inlay, a crown, a veneer, a bridge, and a veneering material for a zirconium oxide restoration substrate, an alumina oxide restoration substrate, or a metal restoration substrate, comprising a leucite glass-ceramic of the invention or a sanidine glass-ceramic or a leucite/sanidine glass-ceramic prepared by the method of the invention.

(63) Fabrication of dental restorations of the present invention can be via several routes.

(64) The glass-ceramic materials can be sintered and glazed on a refractory cast in a vacuum oven. The refractory may then be removed and the restoration etched before adhesive bonding in the dental surgery.

(65) Alternatively, the glasses or glass-ceramic powders can be transferred to a special made steel die and compacted under pressure to produce powder or sintered ingots. These ingots may be extruded into refractory moulds using a heat extrusion technique at temperatures of between about 1000 and about 1150 C., using a 10 minute hold and 15 minutes under pressure. A suitable technique is described in EP 231 773. After removal of the refractory material, the glass-ceramic can next be coloured using stains and glazing in a vacuum oven. Alternatively, the glass-ceramic may be cut back and veneered with a sintered ceramic to further characterise the aesthetics.

(66) The glass-ceramics of the present invention may also be utilised for computer aided design and computer aided manufacturing techniques (CAD-CAM). Glass-ceramic powders may be formed into blanks using heat extrusion or powder compaction and sintering. The morphology and volume fraction of the glass-ceramic of the present invention for use in this technique can be carefully controlled using the processes described herein to allow accurate machining of the dental restoration and to avoid chipping. These blanks may be machined to the desired geometry at the chair side or in the laboratory using currently available CAD-CAM technology. The machined restoration is next stained and glazed in a dental oven/furnace. The glass-ceramics of the present invention may also be printed using 3D printing and using selective binders including: polylactic acid, polymeric additives, sucrose, starch or inorganic binder solutions, followed by sintering in a dental oven/furnace.

(67) Accordingly, the present invention also provides for the use of a leucite glass-ceramic of the invention or a sanidine glass-ceramic or a leucite/sanidine glass-ceramic prepared by the method of the invention in the fabrication of a Dental restoration using one or more of the following processes: sintering, heat pressing, Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) technology, and 3D printing technology.

(68) Embodiments have been described herein in a concise way. It should be appreciated that features of these embodiments may be variously separated or combined within the invention.

EXAMPLES

Example 1

(69) Glass batches consisted of the following components (mole %): SiO.sub.2, 69.3%; Al.sub.2O.sub.3, 10.1%; K.sub.2O, 12.5%; CaO, 2.1%; TiO.sub.2, 0.5%; Na.sub.2O, 4.0%; LiO.sub.2, 1.1%; MgO, 0.5%.

(70) The components were mixed on a jar roll for 2 hours. The batch was transferred to a 90% Pt-10% Rh crucible and heated in an electrical chamber furnace (UAF1600 furnace, Lenton, Hope Valley, UK) at a rate 10 C./min up to 1550 C. and held for 5 hours. The glass was next transferred into an annealing furnace (Tris Burnout furnace, Dentalfarm, Italy) at 500 C. for 2 hours, and then cooled to room temperature. The glass frits were crushed and ground in a ball mill (Pascall Engineering ltd, UK) for 2 hours then screened using a 125 m sieve. One batch of the ground glass was re-homogenised (Example 1a) by a further heating for 2 hours at 1400 C. and then annealed again as previously for 2 hours at 500 C. The glass frit was then re-ground and sieved through a 125 micron sieve. All glasses prepared were clear and transparent. X-ray diffraction was carried out on the glasses using a Siemens D5000 X-ray diffractometer using flat plate geometry. Graphite monochromated Cu K radiation (1=1.54056 and 2=1.54439 ) was used. Data were collected from 5 to 110 two-theta, with a step size 0.02 and a count time of 12 seconds. Glass samples were confirmed as amorphous and leucite free.

(71) The present glasses were explored using Differential Scanning calorimetry (DSC) and the crystallization peak was not sensitive to particle size indicating a bulk crystallization mechanism (FIG. 2). Surface crystallisation was previously thought to be largely responsible for the crystallisation of leucite in aluminosilicate glasses (Hland et al., J Non-Cryst Solids 450 1995; 180: 292-307.25). High speed planetary milling (Pulverisette P7, Fritsch, Idar-Oberstein, Germany) of the glass (0.5-1 hour) to effect surface crystallisation did not lead to an increased number of crystallites. Increase in crystallite number and reduction in median size was achieved after 2 hours high speed milling and may be associated with a change in crystallisation mechanism, but did not translate to increased flexural strengths (192.544 MPa Table III). Fine crystallite size is associated with reduced enamel wear in leucite glass-ceramics and this describes a route to achieve a low wear veneer material via this milling process and change in crystallisation mechanism.

(72) To produce the glass-ceramics, the glasses were heat treated in an electric furnace (RHF 1600, Carbolite, Bamford, UK) using two-step heat treatments. In one embodiment, the un-homogenised glass was heated at a rate of 10 C./min to 610 C. and held for 1 hour, wherein allows the nucleation of leucite; and then ramped to 870 C. and held for 1 hour hold (Example 1, FIG. 4). After heat treatments the glass-ceramic was rapidly air quenched. This produced orientated domains of fiber like crystals in the glassy matrix and a mean (SD) biaxial flexural strength of 199.3 (20.6) MPa (Table IV). Following re-melting (re-homogenisation) of the glass and using the same nucleation and growth temperatures, it was possible to change the microstructure to rosette shaped domains of orientated leucite fibers and a high density of spherical leucite crystals in the glassy matrix (>65% area fraction leucite).

(73) Another embodiment was heat treatment of the re-homogenised glass, wherein the glass was heated at a rate of 20 C./min to 620 C. (or to 591 C.) and held for 1 hour hold, and then ramped to 795 C. and held for 1 hour (or held for 30 mins), followed by air quenching (Example 1a, FIG. 5). A high area fraction of elongated leucite crystals with high aspect ratio together with a high density of fine spherical crystals were crystallized. The elongated fiber-like crystals tend to grow in a preferred manner and form orientated domains. Re-homogenised glasses yielded a much finer microstructure (FIG. 5), which also contained some rosette shaped orientated domains mentioned earlier. These glass-ceramics have high TEC values (19.7 to 21106/K at 100-400 C., Table III), especially at the higher temperature range (400-600 C.) which is around 30106/K. This indicates the high leucite volume fraction of the glass-ceramics, which in agreement with the secondary electron microscope images. All glass-ceramics were confirmed as tetragonal leucite according to high and low temperature XRD data. Additionally, the .sup.27Al triple-quantum magic angle spinning nuclear magnetic resonance experiment has been run for the glass-ceramics (Example 1a). The resulting spectrum in FIG. 3 shows three distinct aluminium sites typical for the leucite phase (Baltisberger et al., J Am Chem Soc 1996; 118: 7209-7214).

(74) It is also possible to change the microstructure of the present glass-ceramics by a short secondary heat treatment to produce a dense dispersal of more spherical type crystals (FIG. 6). The glass-ceramic frit is first ball milled and sieved through a 125 micron sieve to produce glass powder. The powders are vacuum sintered in a dental furnace respectively using a heating rate of 38 C./min. The re-homogenised glass-ceramic specimens were sintered at 1100 C. for 2 mins (FIG. 6). All the sintered specimens showed a high degree of translucency. No microcracking was present in the glassy matrix of the glass-ceramics and the ability to change the microstructure was demonstrated. A high Mean (SD) biaxial flexural strength was produced for this glass-ceramic formulation of 212.3 (28) MPa, Table IV).

(75) In order to induce uniformity of microstructure in the said glass-ceramics, it can be heat extruded. Glass-ceramic powders were transferred to a steel die and compacted under pressure to produce powder or sintered ingots. These ingots may be extruded into refractory moulds using a heat extrusion technique at temperatures of between 1000 and 1150 C., using a 10 min hold and 15 min under pressure. This technique is described in EP 231 773. During this process, the crystals are deliberately grown and the crystal morphology controlled to produce a dense dispersal of uniformly distributed crystals. This eliminates any areas of glassy matrix that may be susceptible to microcracking (FIG. 7). Microcracking is also restricted to the leucite crystals ensuring a strong composite structure which is less likely to fracture and wear the opposing tooth structure. A translucent (refractive index=1.510) and processable glass-ceramic for dental applications is therefore produced. Heat extrusion led to a high Mean (SD) biaxial flexural strength of 227 (35.2) MPa (Table IV). Higher values for reliability (m=11.9) were also achieved by using different glass heat treatments (e.g. 620 C. for a 1-hour hold or 795 C. for a 1 hour-hold) prior to ingot fabrication.

Example 2

(76) Glass reagents batched in the form of silicates, carbonates and oxides consisted of the following components (Mole %):

Example 2

(77) SiO.sub.2, 70.4%; Al.sub.2O.sub.3, 10.3%; K.sub.2O, 12.7%; CaO, 0.5%; TiO.sub.2, 0.5%; Na.sub.2O, 4.0%; LiO.sub.2, 1.1%; MgO, 0.5%.

Example 2a

(78) SiO.sub.2, 69.6%; Al.sub.2O.sub.3, 10.2%; K.sub.2O, 12.5%; CaO, 1.0%; TiO.sub.2, 1.0%; Na.sub.2O, 4.0%; LiO.sub.2, 1.1%; MgO, 0.5%.

Example 2b

(79) SiO.sub.2, 68.2%; Al.sub.2O.sub.3, 10.0%; K.sub.2O, 12.3%; CaO, 2.0%; TiO.sub.2, 2.0%; Na.sub.2O, 3.9%; LiO.sub.2, 1.1%; MgO, 0.5%.

Example 2c

(80) SiO.sub.2, 66.8%; Al.sub.2O.sub.3, 9.7%; K.sub.2O, 12.0%; CaO, 3.0%; TiO.sub.2, 3.0%; Na.sub.2O, 3.8%; LiO.sub.2, 1.1%; MgO, 0.5%.

(81) The components were mixed on a jar roll for 2 hours. The batch was transferred to an 90% Pt-10% Rh crucible and heated in an electrical chamber furnace (UAF1600 furnace, Lenton, Hope Valley, UK) at a rate 10 C./min up to 1550 C. and held for 5 hours. The glass melt was next transferred into an annealing furnace (Tris Burnout furnace, Dentalfarm, Italy) operating at 500 C. for 2 hours, and then slowly cooled down to room temperature. The glass frits were crushed and ground in a ball mill (Pascall Engineering ltd, UK) for 2 hours then screened using a 125 m sieve. The glass powders were re-homogenised by remelting for 2 hours at 1550 C., and annealed for 2 hours at 500 C. and furnace cooled to room temperature. The glass frit was then re-ground and sieved through a 125 micron sieve. All glasses prepared were transparent. X-ray diffraction was carried out on the glasses using a Siemens D5000 X-ray diffractometer using flat plate geometry. Graphite monochromated Cu K radiation (1=1.54056 and 2=1.54439 ) was used. Data was collected from 5 to 110 two-theta, with a step size 0.02 and a count time of 12 seconds. Glass samples were confirmed as amorphous.

(82) In order to crystallise the glass-ceramics the glass powders were heat treated in an electric furnace (RHF 1600, Carbolite, Bamford, UK) using two-step heat treatments. In example 2, the glass powder was heated at a rate of 20 C./min to 593 C. and held for 1 hour, wherein allows the nucleation of leucite; and then ramped to 833 C. and held for 1 hour hold. In example 2a, the glass powder was heated at a rate of 20 C./min to 606 C. and held for 1 hour, then ramped to 868 C. and held for 1 hour hold (Example 2a, FIG. 8). In example 2b, these temperatures comprised 613 C. for 1 hour hold and 903 C. for 1 hour. In example 2c, the heat treatments were 618 C. for 1 hour and 916 C. for 1 hour. Ramp rates were 20 C./min in both instances. After heat treatments the glass-ceramic were air quenched. All glass-ceramics were confirmed as tetragonal leucite according to XRD data.

(83) Example 2a (FIG. 8) demonstrates a largely uniform microstructure, with mostly rounded and few fiber like tetragonal leucite crystals present. Example 2b (FIG. 9) had an increasing fiber-like tetragonal leucite content with high aspect ratio and this was increased for example 2c, which also contained some cross shaped structures (FIGS. 10 and 11). There were no signs of microcracking in the glassy matrix and microcracking was restricted to the leucite crystals. The high TEC values (19.6 to 24.9106/K at 100-400 C., Table III) confirm the high measured area fraction for these glass-ceramics (>54%). The controlled addition of increasing quantities of TiO.sub.2 and CaO of equal molar %, and with K.sub.2O content kept at 12% and above, produced tetragonal leucite glass-ceramics with high reliability (m=9.4-18.9). Example 2a indicates a high Mean (SD) biaxial flexural strength of 224.924.9 MPa and reliability (m=10.9, Table III). These values are increased to a Mean (SD) of 234.8 (15.3) MPa and reliability (m=18.9) for example 2c with the increasing addition of TiO.sub.2 and CaO (3%) of equal molar proportions and with K.sub.2O content kept at 12% and above (Table III). These values for reliability are higher than that found for traditional dental ceramics. Examples 2-2c demonstrate that the microstructure may be varied for these glass-ceramics from a high area fraction of more spherical crystals to those of a high aspect ratio with a resultant effect on the strength, toughness and reliability of the said glass-ceramics. Examples 2, 2a, 2b and 2c may be utilized as machineable or extrusion glass-ceramic materials for dental applications.

Example 3

(84) Glass reagents batched in the form of silicates, carbonates and oxides consisted of the following components (Mole %):

Example 3

(85) SiO.sub.2, 69.3%; Al.sub.2O.sub.3, 10.1%; K.sub.2O, 12.5%; CaO, 2.1%; Nb.sub.2O.sub.5, 0.5%; Na.sub.2O, 3.9%; LiO.sub.2, 1.1%; MgO, 0.5%.

Example 3a

(86) SiO.sub.2, 69.3%; Al.sub.2O.sub.3, 10.1%; K.sub.2O, 12.5%; CaO, 2.1%; TiO.sub.2, 0.5%; Nb.sub.2O.sub.5, 0.5%; Na.sub.2O, 4.0%; LiO.sub.2, 1.1%; MgO, 0.5%.

(87) The components were mixed on a jar roll for 2 hours. The batches were transferred to an 90% Pt-10% Rh crucible and heated in an electrical chamber furnace (UAF1600 furnace, Lenton, Hope Valley, UK) at a rate 10 C./min up to 1550 C. and held for 5 hours. The glass melt was next transferred into an annealing furnace (Tris Burnout furnace, Dentalfarm, Italy) operating at 500 C. for 2 hours, and then cooled down to room temperature. The glass frits were crushed and ground in a ball mill (Pascall Engineering ltd, UK) for 2 hours then screened using a 125 m sieve. The glass powders were remelted for 2 hours at 1550 C., and annealed for 2 hours at 500 C. and furnace cooled to room temperature. Glasses were transparent. The glass frit was then re ground and sieved through a 125 micron sieve. All glasses prepared were transparent. X-ray diffraction was carried out on the glasses using a Siemens D5000 X-ray diffractometer using flat plate geometry. Graphite monochromated Cu K radiation (1=1.54056 and 2=1.54439 ) was used. Data was collected from 5 to 110 two-theta, with a step size 0.02 and a count time of 12 seconds. Glass samples were confirmed as amorphous.

(88) In order to crystallise the glass-ceramics, glass powders were heat treated in an electric furnace (RHF 1600, Carbolite, Bamford, UK) at 20 C./min to 630 C./1 hour and next ramped to 949 C./1 hour then air quenched (Example 3). A translucent glass-ceramic material was produced. For example 3a, the glasses were heated at 20 C./min to 620 C./1 hour and next ramped to 968 C./1 hour then air quenched. Glasses were ground and sieved through a 125 micron sieve. The heat treated glass-ceramics were confirmed as tetragonal leucite by the X-ray diffraction analysis. Using this crystallization process it was possible to demonstrate that a high area fraction (44.1%) of leucite crystals could be synthesized in the glass without the use of titanium dioxide as a nucleating agent (FIG. 12, 14). Titanium dioxide (0.5 mol %) was replaced by niobium oxide (0.5 mol %) in this example. It was previously thought that titanium dioxide needed to be present in the aluminosilicate glass composition to give the copious crystallization of leucite (Holland et al., 1995. J Non-Crystalline Solids 180; 292-307). Differential Scanning calorimetry (DSC) of different glass powder sizes (45, 125, 300 microns) for Example 3 indicated there was little difference in the peak crystallisation temperature (Tp) indicating a bulk crystallisation mechanism. FIG. 12 demonstrates that the microstructure was changed via this process when compared to Example 1 which contained titanium dioxide as a nucleating agent. Glass compositions with a mixed titanium dioxide (0.5 mole %) and niobium oxide content (0.5 mole %) were also produced and led to an increased aspect ratio of some of the leucite crystals (FIG. 13).

Example 4

(89) Glass batches consisted of the following components (mole %):

Example 4

(90) SiO.sub.2, 69.7%; Al.sub.2O.sub.3, 10.6%; K.sub.2O, 12.8%; CaO, 1.5%; TiO.sub.2, 1.3%; Na.sub.2O, 1.9%; LiO.sub.2, 1.6%; B.sub.2O.sub.3, 0.7%.

Example 4a

(91) SiO.sub.2, 69.2%; Al.sub.2O.sub.3, 10.5%; K.sub.2O, 12.7%; CaO, 1.22%; TiO.sub.2, 0.94%; Na.sub.2O, 1.87%; LiO.sub.2, 3.0%; B.sub.2O.sub.3, 0.7%.

Example 4b

(92) SiO.sub.2, 68.4%; Al.sub.2O.sub.3, 10.4%; K.sub.2O, 12.5%; CaO, 1.2%; TiO.sub.2, 0.93%; Na.sub.2O, 1.85%; LiO.sub.2, 4.0%; B.sub.2O.sub.3, 0.7%.

(93) The components were mixed on a jar roll for 2 hours. The batch was transferred to a 90% Pt-10% Rh crucible and heated in an electrical chamber furnace (UAF1600 furnace, Lenton, Hope Valley, UK) at a rate 10 C./min up to 1550 C. and held for 6 hours (or heated up to 1520 C. and held for 5 hours for example 4a). The glass melt was transferred into an annealing furnace (Tris Burnout furnace, Dentalfarm, Italy) operating at 500 C. for 1 hour, then cooled to room temperature. Glasses were then crushed and ground to a powder for 2 hours and then remelted for 2 hours at 1550 C. followed by annealing for 2 hours at 500 C. and furnace cooling to room temperature. The glass frit was then re ground and sieved through a 125 micron sieve. All glasses prepared were transparent. X-ray diffraction was carried out on the glasses using a Siemens D5000 X-ray diffractometer using flat plate geometry. Graphite monochromated Cu K radiation (1=1.54056 and 2=1.54439 ) was used. Data were collected from 5 to 110 two-theta, with a step size 0.02 and a count time of 12 seconds. Glass samples were confirmed as amorphous and leucite free.

(94) In order to crystallise the glass-ceramics, glass powder was heat treated in an electric furnace (RHF 1600, Carbolite, Bamford, UK) at 10 C./min to 618 C./1 hour and next ramped to 1040 C./1 hour (for Example 4) then air quenched. Alternatively (for Example 4a), glass powder was heat treated at 10 C./min to 596 C./1 hour and next ramped to 1040 C./30 mins then air quenched. Glasses were ground and sieved through a 125 micron sieve. The heat treated glass-ceramics were confirmed as tetragonal leucite by the XRD analysis. A mixture of rounded and elongated tetragonal leucite crystals (>54%) dispersed in glassy matrix was crystallised (FIGS. 15, 16). This translucent tetragonal leucite glass-ceramic resulted in a high biaxial flexural strength 229.7 (40.4) MPa (Table III).

(95) High temperature XRD of Example 4 also confirmed it was possible to crystallize (820-1075 C.) a sanidine phase or a mixed leucite-sanidine phase in this glass composition (FIG. 17). High temperature XRD of Example 4a also indicates the presence of sanidine from 750-1000 C. It is possible to crystallize tetragonal leucite/sanidine or a mixed phase leucite-sanidine glass-ceramic in this composition by changes to the crystallization temperatures. The said mixed phase leucite-sanidine glass-ceramic or sanidine glass-ceramic is claimed in this disclosure as the lower thermal expansion characteristics of these glass-ceramics may make them useful as stronger veneering materials for zirconia, alumina or metallic substrates. In order to study surface crystallization effects, the glass powder was also prepared using high speed planetary milling (Pulverisette P7, Fritsch, Idar-Oberstein, Germany) at 1000 rpm for 30 minutes, followed by freeze drying the powder to avoid particle agglomeration. It was possible to crystallize a high area fraction (55.9%) of spherical crystallites uniformly distributed in the glassy matrix using the previous crystallisation parameters (FIG. 18). This material should produce low wear with the tooth enamel according to previous work (Theocharopoulos et al., J Dent Res, 2012; 91, Spec Iss C:29) and sufficient strength as an all-ceramic veneer material.

Examples 5 and 6

(96) Glass reagents batched in the form of silicates, carbonates and oxides consisted of the following components (Mole %):

Example 5

(97) SiO.sub.2, 70.9%; Al.sub.2O.sub.3, 10.0%; K.sub.2O, 10.9%; CaO, 2.1%; TiO.sub.2, 0.5%; Na.sub.2O, 3.9%; LiO.sub.2, 1.1%; MgO, 0.5%.

Example 6

(98) SiO.sub.2, 71.9%; Al.sub.2O.sub.3, 10.1%; K.sub.2O, 9.5%; CaO, 2.0%; TiO.sub.2, 2.2%; Na.sub.2O, 2.6%; LiO.sub.2, 1.1%; MgO, 0.5%.

(99) The components were mixed on a jar roll for 2 hours and transferred to an alumina crucible (CC68, Almath Crucibles Ltd, UK), and heated in an electrical chamber furnace (UAF1600 furnace, Lenton, UK) at a rate of 10 C./min to 1550 C. and held for 5 hours. The molten glass was immediately transferred to a preheated annealing furnace (Tris Burnout furnace, Dentalfarm, Italy) and annealed at 500 C. for 2 hours, followed by furnace cooling. The glass frits were crushed and ground in a ball mill (Pascall Engineering ltd, UK) for 2 hours then screened using a 125 m sieve. In order to produce leucite glass-ceramics the glass was heated using a two-step heat treatment. In one embodiment, the glass was heated at a rate of 10 C./min to 620 C. for a 1 hour hold, then ramped to 920 C. for a 1 hour hold (Example 5, FIG. 19a). A refinement of this leucite microstructure was noticed on extending the nucleation hold to 2 hours (FIG. 19b). It was possible to produce sanidine or leucite/sanidine glass-ceramics by heat treating the said glass in the temperature range of 720-880 C.

(100) For Example 6, the glass was heated at 10 C./min to 670 C. and held for 1 hour and next ramped to 1000 C. and held for 1 hour. After the heat treatment, the glass-ceramic was air quenched and then ground and sieved through a 125 micron sieve. Tetragonal leucite was the only phase detected in both examples 5 and 6. Example 5 illustrates a leucite glass-ceramic with a mixed morphology of both spherical and fiber-like crystals interspersed in the glassy matrix. Domains of orientated fibers are in evidence (FIG. 19b). It was possible to alter the microstructure by the application of a higher crystal growth temperature (960 C.) to produce a more spherical leucite morphology. Some minor areas with a dense dispersal of rod like structures were also present. The glass-ceramic produced was translucent (refractive index=1.507).

(101) Example 6 produced a glass-ceramic which was opaque (refractive index=1.560). Leucite crystal coalescence and large areas of glassy matrix are in evidence (FIG. 20a) compared with Examples 1 and 2. Microcracking is present in the microstructure. The said glass-ceramic illustrated a lower leucite area fraction (33.1%) and this together with the glass and glass-ceramic thermal expansion coefficients (Tables II and III) were associated with the lower K.sub.2O content (33.1%). Glass and glass-ceramic microcracking was in evidence around large areas of crystallisation and this is reflected in the lower biaxial flexural strengths (Table IV). The increased Tg and Dsp in this glass might lead to stress relaxation due to the overlap of cubic/tetragonal leucite transformation temperature. Sparse areas of orientated fiber-like structures in bundles were observed within the microstructure (FIG. 20b) of high aspect ratio. Energy dispersive X-ray analysis revealed these fibers were aluminium and potassium rich compared to the surrounding leucite crystals.

Examples 7 and 8

(102) Glass reagents batched in the form of silicates, carbonates and oxides consisted of the following components (Mole %):

Example 7

(103) SiO.sub.2, 70.6%; Al.sub.2O.sub.3, 8.5%; K.sub.2O, 12.7%; CaO, 2.1%; TiO.sub.2, 0.5%; Na.sub.2O, 4.0%; LiO.sub.2, 1.1%; MgO, 0.5%.

Example 8

(104) SiO.sub.2, 71.0%; Al.sub.2O.sub.3, 8.6%; K.sub.2O, 12.8%; CaO, 1.0%; TiO.sub.2, 1.0%; Na.sub.2O, 4.0%; LiO.sub.2, 1.1%; MgO, 0.5%.

(105) The components were mixed on a jar roll for 2 hours. The batch was transferred to an 90% Pt-10% Rh crucible and heated in an electrical chamber furnace (UAF1600 furnace, Lenton, Hope Valley, UK) at a rate 10 C./min up to 1550 C. and held for 5 hours. The glass melt was next transferred into an annealing furnace (Tris Burnout furnace, Dentalfarm, Italy) operating at 500 C. for 1 hour, and then slowly cooled down to room temperature. The glass frits were crushed and ground in a ball mill (Pascall Engineering ltd, UK) for 2 hours then screened using a 125 m sieve. The glass powders were re-homogenised by remelting for 2 hours at 1550 C., and annealed for 2 hours at 500 C. and furnace cooled to room temperature. The glass fit was then reground and sieved through a 125 micron sieve. All glasses prepared were transparent. X-ray diffraction was carried out on the glasses using a Siemens D5000 X-ray diffractometer using flat plate geometry. Graphite monochromated Cu K radiation (1=1.54056 and 2=1.54439 ) was used. Data were collected from 5 to 110 two-theta, with a step size 0.02 and a count time of 12 seconds. Glass samples were confirmed as amorphous.

(106) Glass-ceramics were produced by using two-step heat treatments of the glasses. In one embodiment, the glass was heated at a rate of 20 C./min to 588 C. for 1 hour hold, then ramped to 820 C. for a 1 hour hold (Example 7). For Example 8, the glass was heated at 20 C./min to 587 C. and held for 1 hour and next ramped to 931 C. and held for 1 hour. After the heat treatment, the glass-ceramics were air quenched and then ground and sieved through a 125 micron sieve. Tetragonal leucite was the only phase detected in Example 7 (FIG. 21) and in Example 8. Reduction in Al.sub.2O.sub.3 content in a series of glasses led to reductions in biaxial flexural strength (Table IV). In Example 7a (Table IV) recovery in biaxial flexural strength (204.7 MPa) was possible via high speed ball milling (0.5 hrs) (Pulverisette P7, Fritsch, Idar-Oberstein, Germany) at 1000 rpm and freeze drying the glass (0.5 hrs) before crystallization heat treatments. A fine (<0.5 microns) leucite crystal microstructure was also achieved via this surface crystallization process (FIG. 21).

Example 9

Determination of Physical Properties of Leucite Glass-Ceramics

(107) A) Thermal Expansion Coefficient

(108) Experimental glass and glass-ceramics frits were cut into blocks (6616 mm) using a diamond saw. The thermal expansion coefficient (TEC) of the glass and glass-ceramics were measured using a differential dilatometer (DIL 402PC, Netzsch Instrument, Germany) in a temperature range of between 20 C. and 1200 C. at a heating rate of 3 C./min with softening point protect. The expansion coefficient was determined in the range of 100-400 C. and between 400-600 C. The results of these experiments indicated a good correlation between the Appen glass predictions and measured TEC of the glasses (Table II).

(109) TABLE-US-00004 TABLE II Results of the differential dilatometry for the experimental glasses. TEC (10.sup.6/K, 100-400 C.) Glass Predicted Measured T.sub.g ( C.) D.sub.sp ( C.) Example 1 10.3 10.4 581.0 676.2 Example 1a 10.0 589.7 669.9 Example 2 10.3 10.3 562.5 667.5 Example 2a 10.3 9.9 578.2 671.5 Example 2b 10.3 10.0 582.7 675.3 Example 2c 10.4 9.98 596.1 657.4 Example 3 9.7 593.1 671.6 Example 3a 9.8 587.0 667.4 Example 4 9.7 9.01 679.0 584.0 Example 4a 9.96 9.5 566.0 654.6 Example 5 9.7 9.8 590.3 667.9 Example 6 8.7 8.7 639.3 727.1 Example 7 10.55 10.5 639 558.0 Example 8 10.53 10.7 553.9 637.6

(110) TABLE-US-00005 TABLE III Results of the differential dilatometry for the experimental glass-ceramics. TEC (10.sup.6/K, Glass-ceramic 100-400 C.) Tg ( C.) Dsp ( C.) Example 1 19.7 478.3 619.8 Example 1a 21.0 480.8 608.9 Example 2 Example 2a 20.23 471.7 611.8 Example 2b 19.61 486.8 657.1 Example 2c 24.87 508.7 658.5 Example 3 Example 3a Example 4 Example 5 18.6 579.7 675.8 Example 6 15.7 627.2 736.5 Example 7 18.72 462.2 583.6 Example 8 19.01 410.5 613.1
B) Biaxial Flexural Strength

(111) The flexural strength of glass-ceramics prepared in Examples 1 to 8 was tested using the biaxial flexural strength test. Glass-ceramic powders were compacted in a steel die and then transferred and sintered in a dental porcelain furnace. Test groups (n=30 per group) were tested by centrally loading the disc specimen on a 10 mm diameter knife-edge support with a 4 mm diameter spherical ball indenter. A thin plastic sheet (0.03 mm) was positioned between the specimen surface and the indenter to distribute the load evenly. The load was delivered by Instron testing machine (5567/H1580, Instron, Buckinghamshire, UK) at a crosshead speed of 1 mm/min until failure. The biaxial flexural strength was calculated using the following equation (Timoshenko and Woinowsky-Krieger, 1959):

(112) ${}_{\max }=\frac{P}{h^2}\left\{\left(1+v\right)\left[0.485*\ln \left(\frac{a}{h}\right)+0.52\right]+0.48\right\}$

(113) Where .sub.max was the maximum tensile stress, P was the measured load at fracture, h was the specimen thickness, a was the radius of the knife-edge support and the Poisson's ratio v of 0.25.

(114) TABLE-US-00006 TABLE IV Results of the Biaxial flexural strength for the experimental glass-ceramics. Biaxial Flexural Characteristic Strength m strength .sub.0 Glass-ceramic Glass-ceramic (MPa (SD) value (MPa) Colour Example 1 199.3 (20.6) 11.5 208.0 Translucent Example 1a 212.3 (28.7) 8.5 224.4 Translucent Example 1a 192.5 (44.0) 5.5 207.5 Translucent (milled 2 hr) * Example 1a 227 (35.2) 7.3 241.8 Translucent (heat extruded) Example 2a 224.9 (24.9) 10.9 235.6 Translucent Example 2b 211.2 (26.7) 9.4 222.5 Translucent Example 2c 234.8 (15.3) 18.9 241 Whitish Translucent Example 4 229.7 (40.4) 6.7 245.7 Translucent Example 4a 205.5 (35.3) 6.7 220.2 Translucent Example 4 168.7 7.9 179.1 Translucent (milled 0.5 hrs)* Example 5 131.7 (9.0) 17.6 135.6 Translucent Example 6 115.3 (10.2) 13.2 119.7 Opaque Example 7 167.7 (28.8) 6.8 179.5 Translucent Example 7 204.7 (38.4) 5.9 220.6 Translucent (milled 0.5 hrs)* Example 8 158.4 (27.5) 6.7 169. Translucent *glass milled before nucleation and growth heat treatments